Load tap changing mechanism having constant duty cycle

ABSTRACT

A load tap changing mechanism wherein the duty of the common drive for the tap changing selector switch and reversing switch is constant in all tap positions.

United States Patent 1 Dusek 1 1 LOAD TAP CHANGING MECHANISM HAVING CONSTANT DUTY CYCLE [75] Inventor: Harold C. Du sek. Menomonee Falls.

Wis.

[73] Assignee: Allis-Chalmers Corporation,

Milwaukee. Wis.

[22] Filed: Feb. 26, 1974 211 Appl. No.: 445.907

[52] U5. C1. 200/11 TC. 200/17 R, 200/153 P,

323/435 R [51} Int. Cl. H01h 19/54 [581 Field of Search 200/11 TC, 17, 17 R, 18,

200/153 P, 153 PA, 153 SC; 323/435, 43.5 R: 335/76; 323/435 R [56] References Cited UNITED STATES PATENTS 2560.002 7/1951 Scale 200/11 TC UX [451 Apr. 1, 1975 2.675.520 4/1954 Scaley 200/11 TC UX 2.811 595 10/1957 Baguhn.. 200/11 TC UX 2,841,662 7/1958 Scaley.... 200/11 TC UX 2.947.838 8/1960 Ureda 200/11 TC UX 3 066 208 11/1962 Fannon, Jr. et 200/11 TC UX 3,153,188 10/1964 McCarty 200/11 TC X 3.612,?86 10/1971 Whitman 323/435 X Primary E.\'aminer-James R. Scott Arlorney, Agent, or Firm-Lee H. Kaiser [57] ABSTRACT A load tap changing mechanism wherein the duty of the common drive for the tap changing selector switch and reversing switch is constant in all tap positions.

8 Claims, 6 Drawing Figures 'LTEF-HEMPR 1:975 3,875 354 seam 1 UF 3 SOURCE SH SH 82 AC SOURCE RS PATENIEDAPR H975 sumzoga LOAD TAP CHANGING MECHANISM HAVING CONSTANT DUTY CYCLE This invention relates to step type load tap changing apparatus such as load tap changing voltage regulators.

BACKGROUND OF THE INVENTION Step type voltage regulators are used extensively in electrical distribution systems to maintain the voltage of a circuit within a predetermined bandwidth. Such voltage regulating apparatus is often referred to as load ratio control equipment and may utilize a load tap changing mechanism of the dial type to change the voltage ratio of autotransformer type stationary induction apparatus without interrupting the load. The voltage regulating apparatus used most frequently employs a motor-operated dial selector switch to change taps and a center-tapped autotransformer reactor to limit current when changing from one tap to the next. A common form of such step type voltage regulating apparatus has an exciting winding connected in shunt to the circuit to be regulated and a tapped series winding connected in series with the load through the tap changing switch.

The load tap changing mechanism may include a dial type tap changing or selector switch having a plurality of stationary contacts arranged in a circle and connected respectively to the taps of the series winding and a pair of movable contacts which are alternately switched and cooperate with the stationary contacts to insert different portions of the series winding in the circuit to be regulated. A reversing switch controlled by the tap changing mechanism is utilized to double the tap range by using the same series tapped winding to provide either a bucking or boosting voltage. The reversing switch is operable to reverse the connections of the tapped series winding with respect to the circuit to be regulated. Most reversing switches include first and second stationary contacts connected respectively to opposite ends of the tapped series winding and a movable contact connected to a supply terminal of the regulator and having first and second positions wherein it respectively engages the first and second stationary contacts to connect opposite ends of the series tapped winding to the supply terminal.

A camming arrangement actuated by the tap changing mechanism is frequently employed to actuate the movable contact of the reversing switch between first and second positions when the movable contacts are switched through the neutral position of the regulator. The switching action of known reversing switches usually starts prior to switching of the tap changer movable contacts from the neutral position to insure that the reversing switch is suitably positioned prior to completion of a tap change to either side of neutral.

The selector switch and the reversing switch of known voltage regulators each have a switch actuating mechanism. the mechanisms having a common drive which has varying duty because the reversing switch mechanical load is only encountered during a tap change when switching through neutral. The torque requirement of the drive motor of known voltage regulating apparatus is greater than that needed to actuate the tap changer dial switch contacts because the reversing switch actuating mechanism is also driven by the same motor. The reversing switch of known voltage regulators is often mounted on the same insulating panel as the tap changer switch stationary contacts with the pivot axis of the reversing switch actuating mechanism offset from the selector switch actuating shaft to permit operation of the tap changing switch and the reversing switch mechanisms by a common drive, and such arrangement can substantially increase the size of the insulating panel and of the voltage regulator.

OBJECTS OF THE INVENTION It is an object of the invention to provide a load tap changer mechanism wherein the duty of the common drive for the selector switch and reversing switch is constant in all tap positions including the position at which the connections of the tapped series winding is reversed. It is a further object of the invention to provide such a load tap changer mechanism wherein a separate reversing switch actuating mechanism is eliminated and no change of duty is encountered by the common drive during the tap change in which the connections of the tapped series winding are reversed.

Another object of the invention is to provide a load tap changer mechanism wherein the pivot point of the reversing switch actuating mechanism is coincident with the axis of the selector switch shaft. Still another object is to provide such a load tap changer wherein the common drive for the selector and reversing switches exerts torque at a single point to both actuate the tap changer switch contacts and operate the reversing switch.

Still another object of the invention is to provide a load tap changer which permits reduction in capacity of the drive motor and of the drive mechanism in comparison to prior art apparatus. A still further object is to provide such a load tap changer mechanism which is simpler in construction, more compact, and lower in cost than prior art apparatus.

SUMMARY OF THE INVENTION In accordance with the invention, a load tap changer has an insulating panel on which the selector switch stationary contacts are mounted in a circle, a carrier for the selector switch movable contacts rotatable to sequentially engage them with the stationary contacts to change taps and insert different portions of the series tapped winding into the circuit to be regulated, and a slip ring reversing switch comprising a slip ring and two spaced arcuate stationary contacts mounted on the insulating panel in spaced concentric relation with the selector switch stationary contacts and a bridging contact transported by the contact carrier and adapted to bridge between the slip ring and the arcuate station ary contacts alternately. The reversing switch slip ring is adapted to be connected to the voltage source supply terminal and the arcuate contacts are adapted to be connected to opposite ends of the tapped series winding so that disengagement of the bridging contact from one arcuate stationary contact and engagement with the other reverses the polarity of the series winding relative to the circuit to be regulated. The common drive for the reversing and selector switches has constant duty in all tap positions since the rotatable contact carrier transports both the selector switch movable contacts and the reversing switch bridging contact.

DESCRIPTION OF THE DRAWING These and other objects and advantages of the invention will be more readily apparent from the following detailed description when considered in conjunction with the accompanying drawing wherein:

FIG. 1 is a schematic circuit diagram of a tap changing voltage regulator embodying the invention;

FIG. 2 is a schematic circuit diagram similar to FIG. 1 but simplified to emphasize the electrical connections accomplished by the reversing switch;

FIG. 3 is a side view partly in cross section of a tap changer embodying the invention;

FIG. 4 is a view taken along line IVIV of FIG. 3;

FIG. 5 is a transverse cross sectional view through the tap changer of FIG. 3 with the selector dial switch removed; and

FIG. 6 is a view taken along line VI-Vl of FIG. 5.

DETAILED DESCRIPTION Referring to FIGS. 1 and 2 of the drawing, a load tap changing voltage regulator of the autotransformer type embodying the invention has a source terminal S adapted to be connected to an alternating current power source, an exciting, or shunt winding SH connected between the source terminal S and the neutral bushing SL, and a tapped series winding SE having a plurality of taps connected to stationary contacts of a dial load tap changing selector switch 10. The dial selector switch 10 may have a plurality of stationary contacts 11 arranged in a circle and a pair of movable contacts and 21 rotatable together about a common axis into sequential engagement with the stationary contacts 11 and being adapted to assume both nonbridging tap positions, wherein both rotatable contacts 20 and 21 engage the same stationary contact 11, and bridging tap positions wherein they engage adjacent stationary contacts. The movable contacts 20 and 21 are adapted to be connected through the windings 23 and 24 of a preventive autotransformer, or reactor 26 to a load bushing L which is coupled to the midtap of the reactor at the junction of windings 23 and 24. Movable contact 20 may be joined to a resilient contact finger 27 (see FIG. 4) which continuously resiliently engages a conductive collector ring 28 electrically connected to reactor winding 23, and movable contact 21 may be joined to resilient contact fingers 29 which continuously resiliently engage a conductive stud 30 electrically connected to reactor winding 24.

The embodiment of the invention illustrated in the drawings utilizes substantially the same arrangement of selector switch stationary contacts that is commonly employed to accomplish buck or boost of voltage in a plurality of steps (e.g., nine stationary contacts employed to effect i l0 percent buck or boost in sixteen /a percent steps) when the movable contacts are rotated through almost a complete revolution in either direction from the neutral position. but which arrangement requires a separate actuating mechanism for the reversing switch. The illustrated embodiment utilizing such conventional stationary contact arrangement only moves rotatable contacts 20 and 21 through approximately one-half revolution in each direction from the neutral position to respectively effect a voltage buck or boost in eight steps. Source bushing S is coupled to selector switch neutral stationary contact N and may be connected by reversing switch RS alternatively to opposite ends of series tapped winding SE as shown in FIGS. 1 and 2. Opposite ends of series winding SE are connected to stationary contacts 1 and 5 which are the furthest removed from neutral stationary contact N in the clockwise and counterclockwise directions respectively. Both movable contacts 20 and 21 engage stationary contact N when the voltage regulator is in the neutral position and may be actuated clockwise through eight steps to effect a voltage boost or counterclockwise through eight steps to accomplish a decrease (buck) in voltage. One intermediate tap in series winding SE is connected to stationary contact 2 adjacent to neutral stationary contact N in the counterclockwise direction and is also connected to the third stationary contact 2 removed in the clockwise direction from contact N as shown in FIG. 1. The middle tap of series winding SE is connected to each of the second stationary contacts 3 removed from neutral contact N in both the clockwise and counterclockwise directions. Another intermediate tap in series winding SE is connected to the stationary contact 4 adjacent to neutral contact N in the clockwise direction and is also connected to stationary contact 4 which is the third stationary contact removed from neutral contact N in the counterclockwise direction.

DIAL SELECTOR SWITCH The selector switch stationary contacts 11 are arcuately spaced apart and each is preferably a generally.

U-shaped metallic member (see FIGS. 3 and 4) having one leg brazed to a stud 31 extending through an insulating panel 32 and which may be electrically connected to a tap of series winding SE. Each stationary contact 1-5 may extend arcuately approximately 27 and neutral stationary contact N may extend approximately 45. Selector switch rotatable contacts 20 and 21 are mounted in arcuately spaced relation on an insulating contact carrier 37 which is rotatable about a metallic collector hub 30 affixed to insulating panel 32 by metallic studs (not shown) that are adapted to be electrically connected to winding 23 of the preventive autotransformer 26. The movable contacts 20 and 21 are preferably spaced by an angle such as 20 which is equal to the angle through which contact carrier 37 rotates during each tap change and is greater than the angle between adjacent stationary contacts 11 to permit them to assume bridging tap positions.

Rotatable contact 20 may comprise a metallic bar 40 affixed to contact carrier 37; a pair of opposed resilient conductive contact fingers 41 secured to the radially outer end of bar 40 and adapted to engage opposite sides of one radially inward extending leg of each U- shaped stationary contact 11; and resilient conductive contact finger 27 secured to the radially inner end of bar 40 and adapted to continuously resiliently engage conductive slip ring 28 which is affixed to insulating panel 32 by studs (not shown) and which is adapted to be electrically connected to winding 24 of preventive autotransformer 26. Rotatable contact 21 may similarly comprise a metallic bar 44 affixed to insulating contact carrier 37; a pair of opposed resilient conductive contact fingers 41 secured to the radially outer end of bar 44 and adapted to engage opposite sides of the radially inward extending leg of each U-shaped stationary contact 11; and the pair of resilient contact fingers 29 secured to the sides of bar 44 adjacent its radially inner end adapted to maintain continuous electrical engagement with collector stud 30 which is adapted to be electrically connected to winding 23 of preventive autotransformer 26.

SWITCH OPERATING MECHANISM The ends of the legs of a generally U-shaped mounting frame 51 are affixed to insulating panel 32. A stationary circular index plate 53 having circumferentially spaced notches 54 about its periphery is affixed by bolts to mounting frame 51 in spaced parallel relation to insulating panel 32. A main shaft 55 extending axially through index plate 53 may comprise a bolt extending through an aperture in frame 51 and through a bushing 56 abutting the side of frame 51 opposite the bolt head with a nut 57 threadedly engaging the end of bolt 55. A sprocket wheel 58 having external teeth is rotatable about bushing 56 and is powered by a chain drive 59 actuated by a motor sprocket 60 mounted on the shaft of a reversible motor M which is energized to turn sprocket wheel 58 and thus rotate movable contacts 20 and 21 in a direction to raise or lower the voltage supplied through contacts 20 and 21 to load bushing L. Motor M may be-controlled in known manner to rotate sprocket wheel 58 at uniform speed to change taps in a direction to maintain the voltage supplied to load bushing L within a desired voltage bandwidth.

An actuating or interlock disk 62 rotatable about bushing 56 is coaxial with and disposed between sprocket wheel 58 and contact carrier 37 and carries a driving pin 63 extending parallel to the axis thereof which fits on a cooperating groove 64 in contact carrier 37 so that interlock disk 62 rotates with contact carrier 37 and thus rotates movable contacts 20 and 21. Pin 63 constitutes a common drive point for the selector and reversing switches. Sprocket wheel 58 is connected to interlock disk 62 by two spring assemblies 65 positioned symmetrically about main shaft 55. One end of each spring assembly 65 is attached to a pin 67 affixed to sprocket wheel 58 and its other end is attached to a pin 68 affixed to interlock disk 62 so that spring assemblies 65 are biased in both directions of rotation of sprocket wheel 58.

Actuating disk 62 is normally held stationary by a latch 70 which is pivotally attached to disk 62 and extends through an arcuately elongated aperture 71 in sprocket wheel 58 (see FIG. 6) and engages one of the notches 54 in index plate 53. At the beginning of a tap change sprocket wheel 58 is in a balanced position relative to interlock disk 62 so that neither spring assembly 65 is loaded more than the other and latch 70 is held in a notch 54 in index plate 53 by a spring 72 (see FIG. 5). As sprocket wheel 58 is rotated at slow uniform speed by the drive motor M and chain drive 59, the spring assemblies 65 are oppositely biased since like ends of the spring assemblies are attached to the rotating sprocket wheel 58 while the other ends are attached to actuating disk 62 which is prevented from turning by the engagement of latch 70 within a notch 54. After a predetermined rotation of sprocket wheel 58, a cam surface 73 partially defining aperture 71 lifts latch 70 from the notch 54 in index plate 53, thereby permitting one ofthe biased spring assemblies 65 to rapidly rotate actuating disk 62, and thus snap contact carrier 37 and movable contacts 20 and 21 with rapid motion to the succeeding tap position.

A stationary stop 74 mounted on frame 51 at a position approximately between stationary contacts 1 and 5 is in the path of a rotatable limit member 75 affixed to and extending radially from sprocket wheel 58 at a position approximately in radial alignment with movable contacts 20 and 21 and limits rotation of carrier 37 and movable contacts 20 and 21 transported thereby so they cannot rotate beyond stationary contacts 5 and 1 in the counterclockwise and clockwise directions respectively.

REVERSING SWITCH The slip ring reversing switch RS preferably comprises aconductive collector, or slip ring 76 affixed by studs (not shown) to insulating panel 32 in concentric surrounding relation to collector ring 28 and which studs are adapted to be electrically connected by suitable conductors to the source bushing 5 and to the neutral stationary contact N; a pair of arcuate, almost semicircular, conductive collector bars 77 and 78 mounted in a circle on insulating panel 32 with their ends spaced apart and concentric with and spaced radially outward from slip ring 76 by studs (not shown) extending through panel 32 and which are adapted to be connected by suitable conductors to opposite ends of series winding SE so that collector bars 77 and 78 are electrically connected respectively to stationary contacts 1 and 5; and a conductive metallic block 80 secured to contact carrier 37 so as to rotate therewith and carrying at its radially inner end a resilient contact finger 81 adapted to maintain continuous electrical engagement with slip ring 76 as contact carrier 37 rotates and also carrying at its radially outer end a resilient contact finger 82 adapted to respectively electrically engage arcuate collector bars 77 and 78 as contact carrier 37 rotates.

Slip ring reversing switch RS comprising bridging bar 80 and contact fingers 81 and 82 bridges between slip ring 76 and arcuate collector bars 77 or 78 to connect source terminal S and neutral contact N respectively to opposite ends of series tapped winding SE and thus reverse the polarity of the series winding SE with respect to the circuit to be regulated, i.e., to selectively increase or decrease the voltage supplied to load bushing L. In the tap position shown in FIGS. 1 and 4, movable contacts 20 and 21 both engage neutral stationary contact N and reversing switch RS connects collector bar 78 to slip ring 76 so that source terminal S is electrically connected to the end of series winding SE coupled to stationary contact 1. Successive energizations of drive motor M accomplishing two tap changing cycles are necessary to switch through the neutral position, assuming that the voltage sensing means (not shown) first energizes drive motor M to effect clockwise rotation of movable contacts 20 and 21 to the position shown in FIGS. 1 and 4 wherein they engage stationary contact N and then again energizes drive motor M to accomplish a second tap change. Contact carrier 37 will be rotated clockwise during such second tap change cycle to a position wherein both movable contacts 20 and 21 still engage neutral stationary contact N, and reversing switch movable bridging contact 80, 81, 82 will be rotated to a position shown in dotted lines wherein slip ring 76 is connected through the bridging contact to arcuate collector bar 77 to thereby disconnect source bushing S from the end of series winding SE coupled to stationary contact I and to connect source bushing S to the opposite end of series winding SE which is connected to stationary contact 5, thereby reversing the polarity of series winding SE. It will be appreciated that both movable contacts 20 and 21 engage neutral contact N when the reversing switch RS is operated to disengage arcuate contact 78 and to engage arcuate contact 7750 that no current is interrupted and no arcing occurs. If the voltage supplied to load bushing L is still outside the bandwidth, the voltage sensor means may again energize drive motor M to rotate drive sprocket wheel 58 and actuate contact carrier 37 further in the clockwise direction to engage movable contact 21 with stationary contact 4. During further boosting tap changes reversing switch RS will continuously bridge slip ring 76 t arcuate collector bar 77 to maintain the connection between source terminal S and the end of series winding SE coupled to stationary contact 5.

it will be noted that the pivot axis of reversing switch RS is the axis of rotation of contact carrier 37 which is also the axis of rotation of selector switch movable contacts and 21. Inasmuch as reversing switch bridging contact 80 and selector switch movable contacts 20 and 21 are mounted on the same contact carrier 37, the duty imposed on the switch drive means is constant in all tap positions. Consequently, the capacity of the drive motor M and the drive mechanism can be materially decreased in comparison to prior art devices wherein a separate actuating mechanism was required for the reversing switch which was only operated during the tap change when switching through the neutral position. Inasmuch as the reversing and selector switches have a common axis of rotation, the insulating panel may be reduced in size in comparison to prior art devices, 'and also the voltage regulator can be more compact. It will also be appreciated that the common drive for the selector and reversing switches of the disclosed invention exerts torque at a single drive point, i.e., at the actuating pin 63, to operate both the selector switch and the reversing switch. Further, the symmetrical arrangement of insulating panel 32 with a common axis for the reversing and selector switches permits electrical clearance symmetry.

Although the disclosed embodiment only permits eight operating steps in raising or in lowering the voltage, the invention is not so limited and other embodiments permit voltage boost or voltage buck in sixteen or any desired number of steps.

It should be understood that I do not intend to be limited to the particular embodiment shown and described for many modifications and variations thereof will be readily apparent to those skilled in the art.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In combination with a tap changing selector switch having a plurality of stationary contacts arranged in a circle adapted to be electrically connected to taps of a tapped electrical winding, a pair of movable contacts, and a carrier for said movable contacts rotatable about an axis through the center of said circle to sequentially engage said movable contacts with said stationary contacts;

a reversing switch including a stationary conductive slip ring concentric with said circle and adapted to be connected to an electrical power source, first and second stationary conductive arcuate contacts adapted to be electrically connected to opposite ends of said winding and disposed in circumferentially spaced relation concentric with said selector switch stationary contact circle and said slip ring,

and a conductive bridging contact transported by said carrier in continuous electrical engagement with said slip ring and alternately electrically engaging said first and second stationary arcuate contacts as said carrier rotates to thereby alternately bridge between said slip ring and said first and second stationary arcuate contacts.

2. A tap changing arrangement including an insulating panel, a plurality of selector switch stationary conductive contacts mounted in a circle on said panel, a pair of selector switch movable contacts, a carrier for said movable contacts rotatable about an axis through the center of said circle to sequentially switch said movable contacts into engagement with said stationary contacts, a reversing switch stationary conductive slip ring mounted on said panel concentric with and radially inward from said circle of stationary contacts, first and second reversing switch conductive stationary arcuate contacts mounted in spaced apart relation on said panel concentric with said slip ring, a conductive bridging contact mounted on said carrier in continuous electrical engagement with said slip ring and to alternately electrically engage said first and second stationary arcuate contacts as said carrier rotates and thereby alternately bridge between said slip ring and said first and second stationary arcuate contacts, and reversible drive means for intermittently indexing said rotatable carrier with a quick step movement to switch said movable contacts into sequential engagement with said selector switch stationary contacts.

3. A tap changing arrangement in accordance with claim 2 wherein said selector switch stationary contacts are adapted to be electrically connected to taps of a tapped electrical winding, said reversing switch first and second stationary contacts are adapted to be electrically connected to opposite ends of said tapped winding, and said slip ring is adapted to be electrically connected to an electrical power source.

4. A tap changing arrangement in accordance with claim 3 wherein one of said selector switch stationary contacts is isolated from said electrical winding and adapted to be electrically connected to said slip ring and is of sufficient circumferential length so that both of said pair of movable contacts may remain in engagement therewith during one quick step movement by said drive means when the selector switch is in the neutral position, the arrangement being such that said bridging contact disengages said reversing switch first arcuate contact and engages said reversing switch second arcuate contact during the said one quick step movement when both movable contacts engage said one selector switch stationary contact.

5. In the combination of claim 4 wherein said one selector switch stationary contact is the neutral contact and the selector switch stationary contacts furthest removed in both clockwise and counterclockwise directions from said neutral contact are adapted to be electrically connected to opposite ends of said tapped electrical winding.

6. A tap changing arrangement in accordance with claim 2 and including first and second conductive circular collector members mounted in spaced relation on said panel radially inward from and concentric with said circle of selector switch stationary contacts, and resilient means for maintaining said selector switch movable contacts in continuous electrical engagement 9 respectively with said first and second circular collector members as said carrier rotates.

7. A tap changing arrangement in accordance with claim wherein successive selector switch stationary contacts disposed in the approximately 180 arc in the clockwise direction from said neutral contact are electrically connected to sequential taps of said winding beginning t'rom the winding and connected to the stationary contact furthest removed in the counterclockwise direction from said neutral contact. and successive selector switch stationary contacts disposed in the approximately l80 arc in the counterclockwise direction from said neutral contact are electrically connected to sequential taps of said winding beginning from the driving and driven members. 

1. In combination with a tap changIng selector switch having a plurality of stationary contacts arranged in a circle adapted to be electrically connected to taps of a tapped electrical winding, a pair of movable contacts, and a carrier for said movable contacts rotatable about an axis through the center of said circle to sequentially engage said movable contacts with said stationary contacts; a reversing switch including a stationary conductive slip ring concentric with said circle and adapted to be connected to an electrical power source, first and second stationary conductive arcuate contacts adapted to be electrically connected to opposite ends of said winding and disposed in circumferentially spaced relation concentric with said selector switch stationary contact circle and said slip ring, and a conductive bridging contact transported by said carrier in continuous electrical engagement with said slip ring and alternately electrically engaging said first and second stationary arcuate contacts as said carrier rotates to thereby alternately bridge between said slip ring and said first and second stationary arcuate contacts.
 2. A tap changing arrangement including an insulating panel, a plurality of selector switch stationary conductive contacts mounted in a circle on said panel, a pair of selector switch movable contacts, a carrier for said movable contacts rotatable about an axis through the center of said circle to sequentially switch said movable contacts into engagement with said stationary contacts, a reversing switch stationary conductive slip ring mounted on said panel concentric with and radially inward from said circle of stationary contacts, first and second reversing switch conductive stationary arcuate contacts mounted in spaced apart relation on said panel concentric with said slip ring, a conductive bridging contact mounted on said carrier in continuous electrical engagement with said slip ring and to alternately electrically engage said first and second stationary arcuate contacts as said carrier rotates and thereby alternately bridge between said slip ring and said first and second stationary arcuate contacts, and reversible drive means for intermittently indexing said rotatable carrier with a quick step movement to switch said movable contacts into sequential engagement with said selector switch stationary contacts.
 3. A tap changing arrangement in accordance with claim 2 wherein said selector switch stationary contacts are adapted to be electrically connected to taps of a tapped electrical winding, said reversing switch first and second stationary contacts are adapted to be electrically connected to opposite ends of said tapped winding, and said slip ring is adapted to be electrically connected to an electrical power source.
 4. A tap changing arrangement in accordance with claim 3 wherein one of said selector switch stationary contacts is isolated from said electrical winding and adapted to be electrically connected to said slip ring and is of sufficient circumferential length so that both of said pair of movable contacts may remain in engagement therewith during one quick step movement by said drive means when the selector switch is in the neutral position, the arrangement being such that said bridging contact disengages said reversing switch first arcuate contact and engages said reversing switch second arcuate contact during the said one quick step movement when both movable contacts engage said one selector switch stationary contact.
 5. In the combination of claim 4 wherein said one selector switch stationary contact is the neutral contact and the selector switch stationary contacts furthest removed in both clockwise and counterclockwise directions from said neutral contact are adapted to be electrically connected to opposite ends of said tapped electrical winding.
 6. A tap changing arrangement in accordance with claim 2 and including first and second conductive circular collector members mounted in spaced relation on said panel radially inward from and concentric wIth said circle of selector switch stationary contacts, and resilient means for maintaining said selector switch movable contacts in continuous electrical engagement respectively with said first and second circular collector members as said carrier rotates.
 7. A tap changing arrangement in accordance with claim 5 wherein successive selector switch stationary contacts disposed in the approximately 180* arc in the clockwise direction from said neutral contact are electrically connected to sequential taps of said winding beginning from the winding and connected to the stationary contact furthest removed in the counterclockwise direction from said neutral contact, and successive selector switch stationary contacts disposed in the approximately 180* arc in the counterclockwise direction from said neutral contact are electrically connected to sequential taps of said winding beginning from the winding and connected to the stationary contact furthest removed in the clockwise direction from said neutral contact and are electrically commoned with said stationary contacts disposed diametrically opposite thereto.
 8. A tap changing arrangement in accordance with claim 2 wherein said selector switch movable contact carrier includes a driven insulating member and said quick step drive means includes a driving member rotatable about an axis through the center of said selector switch stationary contact circle and a driving pin disposed radially outward from said axis and coupling said driving and driven members. 